1. Field of the Invention
The present invention relates to a switching booster power circuit.
2. Description of Related Art
A switching booster (step-up) power circuit includes a booster circuit and a switching control between a direct-current power source and a capacitor. The booster circuit has a switching element, and the switching control controls on/off of the switching element of the booster circuit.
JP-A-7-123708 discloses a booster circuit, which is constructed by a coil, a diode and a switching element, e.g., transistor, for example. The booster circuit gradually increases voltage, so that the capacitor of the switching booster power circuit can be charged with a larger amount of energy.
That is, the switching booster power circuit charges the capacitor, when activated. Immediately after the activation is started, an equivalent impedance of the capacitor is approximately zero, because the capacitor has not been charged yet. Therefore, at the activation, a rush current passes through the switching booster power circuit. The rush current represents a relatively large current, which temporarily passes when direct-current electricity is applied to an electric load (current consumer) having a capacitance.
Further, immediately after the switching booster power circuit is activated, input voltage of the direct-current power source is gradually increased to be a maximum voltage value of the direct-current power source. The rush current passes mainly in a period before the input voltage becomes equal to the maximum voltage value.
When the transistor (switching element) is turned on while the rush current passes through the circuit, the rush current passes through the transistor. In this case, the transistor may be damaged. Therefore, in a conventional art, the switching element has a relatively large capacity to withstand the rush current.
Further, a control device, e.g., microcomputer, for controlling activation of the switching control may be disposed at a subsequent part of the capacitor to be charged. In this case, the microcomputer can prevent the switching element from being turned on while the rush current passes through the circuit.
However, when the switching element has the relatively large capacity to withstand the rush current, the switching element may be expensive. Further, when the microcomputer controls the switching control, a start-up time and a self-check time of the microcomputer are needed before the controlling of the switching control is started. In this case, the starting of the controlling of the switching control may be too much delayed even after the rush current passes through the circuit. Then, start of voltage increasing by the booster circuit may be delayed.
Further, the microcomputer is activated only when a rated voltage is supplied thereto, after electricity is supplied from the direct-current power source and the booster circuit. That is, a step-down circuit decreases voltage supplied from the direct-current power source and the booster circuit to the rated voltage, and the microcomputer is activated with the decreased voltage (rated voltage). Therefore, time is more needed for activating the microcomputer, so that the start of the voltage increasing by the booster circuit may be much delayed. Here, the rated voltage represents a voltage with which electric equipment normally operates.
Further, the activation of the microcomputer is used as a reference timing for the switching control to start controlling the switching element. That is, voltage decreased by the step-down circuit is used as a reference voltage for the switching control to start controlling the switching element. Because voltage supplied from the direct-current power source is not directly used as the reference voltage for the switching control to start controlling the switching element, the start of the controlling of the switching element may be more delayed.
Furthermore, even after the rated voltage is supplied to the microcomputer, the microcomputer is activated only when the rated voltage becomes stable after reset. Thus, the start of the controlling of the switching element may be more delayed. That is, in the case where the microcomputer controls the switching control, the start of the controlling of the switching element may be too much delayed.